My wife owns the CX4 and is quite content with it. We did not step up to newer models.

However, in my experience, corner softness has been a persevering problem for the R as well as for the CX range. We had several cameras sent to the repair center, often with only little success, and eventually replaced with a new sample -- mostly also not with the result that we had hoped for...

With respect to high ISO, we usually do not go higher than ISO 800. Although high ISO performance has improved over the years in this camera range, I would use ISO 1600 only in cases of "emergency". (My wife uses the MY settings for ISO 400 and 800 -- usually she uses ISO auto.)

Nonetheless, my wife likes her CX4, its compactness, useful zoom range, and its macro (if you take macro shots...).